If someone has a valid visa on his/her old passport, then it is advisable to send the old passport along with the new one. There is another scenario in which if old passport doesn't have any valid visa but the passport holder has a travel history which can be confirmed from the old passport, then it is also recommended to send old passport along with the new one. In any of above cases, if passport holder doesn't send the old passport along with the new one, it is not going to put any negative impact on the application; it will be on the discretion of visa officer, he/she might ask for the old passport or just stamp the PR visa on the new passport.
If there is no valid visa on the old passport or there is no travel history recorded on the old passport, then it is not required to send the old passport along with the new one.